Fresh vegetables have a short harvest period and have the tendency to spoil quickly if not stored under refrigerated conditions. Some practical methods for shelf-life extension of fresh vegetables have found significant commercial application. These methods include modified atmosphere packaging (MAP)/controlled atmosphere packaging (CAP), air drying, freeze drying, vacuum drying, individually quick freezing (IQF), and canning. With MAP/CAP, shelf-life is increased for a few days with refrigeration from packaging to consumption because microbial growth is dependent on the storage conditions (Oliveira, et al., 2015). The refrigeration increases the cost of shipping and storing MAP/CAP products.
Additionally, when fresh vegetables are peeled and cut, their shelf lives are reduced immensely due to the high probability of enhanced enzymatic activity and microbial growth. Methods used for shelf life extension of fresh cut vegetables also include air-drying, freeze-drying, vacuum drying, IQF, and canning. Dehydration using air is a commercially used practice but the collateral damage to the taste, nutritional quality, and structural deformation due to shrinkage of the product make the process unattractive. Although freeze-drying and vacuum drying provide better quality products, they suffer from the limitations in commercial application of being very expensive (Nijhuis, et al., 1998).
U.S. Pat. No. 4,988,523 describes a sulphite-free, anti-browning coating for fresh-peeled potatoes. The patent describes a dipping or spraying process employing a mixture of the commercial preservative SPORIX™ and citric acid. It can protect the color of fresh-peeled potatoes (whole or quartered) for up to 12 days without the use of sulphites and without imparting a sulphite-taste to the potato. However, the process requires that the potatoes be stored under refrigerated conditions (1-4° C.).
Some other techniques which are commercially practiced include canning and IQF. When compared to fresh produce, frozen vegetables that are later cooked have relatively poor texture. Furthermore, the cost of energy is high for IQF. Similarly, the thermal treatment during canning deteriorates the textural and nutritional qualities of the product.
U.S. Pat. No. 4,336,273 discloses a process for preserving vegetables by exposing them to a spray, vapor, or pool of organic compounds, such as aldehydes, amides, esters, hydrocarbons, halogenated hydrocarbons, ketones, etc., for 10-20 minutes between 0° to 75° C. Preservation with this technique requires washing or rinsing the organic compound from the vegetable prior to heat processing. The organic compound must be carefully and completely washed away. The flavors of the vegetables are adversely affected by overexposure to the organic compounds.
U.S. Pat. No. 5,486,369 describes a combination of blanching and drying to achieve a lower water activity to avoid growth of micro-organisms. The raw potato pieces are first blanched in a heated infusion solution containing one or more sugars and sodium chloride to infuse sugar and sodium chloride into the potato pieces. The potato pieces are dried by subjecting the pieces to heat for a sufficient time and temperature to reduce the water activity of the pieces to below 0.85. These potato pieces can be stored in ambient room temperatures and don't require any special packaging. The water activity below 0.85 along with high sodium and sugar concentration renders a final product which lacks desired characteristics both from a texture and taste point.
U.S. Published Patent Application No. US-2015/0010691 describes a chemical preservation methodology. It is based on a preservative solution comprising of water, sodium chloride, citric acid, ascorbic acid, calcium chloride, sodium acid phyrophosphate, potassium sorbate and a protein based composition and applying the preservative solution to fresh cut potatoes. These products need to be kept under refrigeration for the period of their shelf life.
EP 0811323 describes another chemical preservation technology that uses the application of an aqueous solution in which a combination of two antioxidizing agents participate, specifically potassium metabisulfite and sodium disulfite, as well as a humidifying agent, specifically sorbitol, and a synergetic agent, specifically ascorbic D.L. acid, wherein prior to immersing the peeled potatoes in said aqueous solution there exists an intermediate humidification phase consisting of a water bath lasting for a period of approximately 30 minutes. The final product can be stored under refrigeration for 21 days.
Another method has been performed using SCCO2 with ethanol as a processing aid (co-solvent) for the dehydration of Carrots (Fryer, Norton, bakalis, & bridson, 2007). The method describes a batch type system with continuous CO2 flow mechanism operating at definite process parameters (200 Bar; 60° C.). During the process the product is placed without a pre-package into the high pressure chamber. According to the research, the texture of the product is better than the air dried products but not as good as its freeze-dried counterpart. This process is highly time consuming and it removes volatile compounds, resulting in flavor and color loss in the final product. It also states the process of SCCO2 drying to be zero order and air-drying to be a first order process which is not true in this research.
Other related research publications show that inactivation of microbial and enzymatic activity can be accomplished by treatment with SCCO2 (Wimmer & Zarevucka, 2010) which is also a batch type system. U.S. Pat. No. 7,108,832 describes the application of SCCO2 for the sterilization of thermally or hydrolytically sensitive, medically-important materials including biodegradable and other medical polymers, tissue for implantation or transplantation, medical equipment, drugs and drug delivery systems. The product is processed in a system with a non-continuous CO2 flow mechanism.
Similar is U.S. Published Patent Application No. US-2012/0288614 A1, which uses supercritical carbon dioxide to achieve inactivation of the certain micro-organisms and enzymes in liquid, semi-solid and solid foods within a batch type apparatus with a non-continuous CO2 operation. The operating temperature during the process is 40° C. wherein the product is packaged in a gas permeable bag (breathable) and is placed inside the pressure vessel. The vessel is pressurized to get the carbon dioxide to supercritical state and the product is exposed to the supercritical carbon dioxide for various time periods after which it is slowly depressurized to avoid the breakdown of the cellular structure of the product.
The present invention is directed to overcoming these and other deficiencies in the art.